Scientists can measure the rates at which evolutionary changes happen. By measuring changes in the frequencies of existing genes and observing mutations, they can describe the evolutionary process exactly. They can see in the laboratory how selection can result in changes in gene frequencies.
Here's a favorite example of mine to show how witnessing evolution in action is possible: antibiotic resistance. Scientists can observe how the frequencies of genes that make a microbe resistant to antibiotics increase when the environment changes to include an antibiotic. They take a beaker full of microbes and add some antibiotics. Then — big drumroll here — they come back the next day and find that the frequency of genes conferring resistance to antibiotics is much higher than it was before the antibiotics were added. Why? Because all the bacteria that didn't have antibiotic-resistance genes died and the ones that did have antibiotic resistant genes reproduced a lot! Hence, the favored trait — resistance to antibiotics — increased over time. You can read more about antibiotic resistance in Chapter 17.
Mutations don't occur in response to environmental change. Instead, the mutations already exist and are favored in the new environment. Beakers full of bacteria tend to contain mutants that are resistant to antibiotics even when no antibiotics are around. Toss an antibiotic into the mix, and the mutation gets its chance to shine by enabling the bacteria to survive in the presence of the antibiotic.
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